1. Technical Field of the Invention
This invention relates to a method and apparatus for detecting a target analyte from a sample fluid. More specifically, the target analyte is separated from the sample fluid through interaction with magnetic capture agents, and application of an electromagnetic field transports the separated analyte for detection. In this manner, fluid reagents used in the assay need not be transported or removed from the device when the assay is being conducted.
2. Brief Description of the Related Technology
Specific binding assays have found wide-spread use in the detection of target analytes contained in sample fluids, particularly in the field of biomedical research. The target analytes which are detectable using such specific binding assays include hormones, vitamins, blood group antigens, viruses (e.g., rotavirus), microorganisms (e.g., Streptococcus, Salmonella, Treponema, and the like), therapeutic drugs (e.g., theophylline), abused drugs (e.g., canabinoids, morphine, and the like), and pesticides (e.g., arachor). In addition, the fields of forensic detection (e.g., animal vs. human blood), blood typing rare blood groups, and environmental monitoring (e.c., polychlorinated biphenyls) also benefit from such specific binding assays.
Specific binding assays are of two types: homogeneous specific binding assays and heterogeneous specific binding assays. Using either assay, a target analyte becomes bound to a capture agent thereby forming a complex and a detectable response from that complex is determined. In homogenous specific binding assays, the detectable response of unbound or free analytes differs from that of the analyte bound in the complex. Hence, the bound analyte can be distinguished from the unbound analyte and detection of the target analyte may be accomplished selectively without requiring physical separation of the bound analyte from the sample. See e.g., S.H. Jenkins, "Homogeneous Enzyme Immunoassay", J. Immun. Meth., 150, 91-97 (1992).
In addition, homogeneous specific binding assays are relatively simple to perform and are easily adaptable to automated analyzers. Previous commercial analyzers that have been used in the clinical laboratory include the ACA.TM., a discrete analyzer from Du Pont de Nemours and Co., Wilmington, Del.; the SMAC.TM., an analyzer based on linear flow of analytes made by Technicon (now Miles Diagnostics, Tarrytown, N.Y.) and the Cobas.TM., which is an analyzer based on centrifugal forces which transport reagents and is available from Roche Diagnostics, Branchburg, N.J.
Such homogeneous specific binding assays do, however, require several intermediate steps, each of which typically includes the use of separate reagents. In addition, although the lack of a separation step is beneficial from an ease of performance standpoint, it is detrimental to the extent that the sensitivity and specificity of certain assays may be hindered. Sensitivity and specificity may be compromised due to the presence of background interference, the cause of which ordinarily would be removed during a separation step.
In contrast to homogeneous specific binding assays, heterogeneous specific binding assays exhibit identical responses from the bound analyte and the unbound analyte. A separation step is therefore required in order to obtain meaningful detection results. Once the bound analyte is separated from the unbound analyte, the heterogeneous specific binding assay demonstrates high sensitivity and analyte specificity. An example of such a heterogeneous specific binding assay is described in C. Tu et al., "Ultrasensitive Heterogeneous Immunoassay Using Photothermal Deflection Spectroscopy", Anal. Chem., 65, 3651-53 (1993).
Previously, magnetic particles have been used in heterogeneous specific binding assays. In fact, they have been reported as useful for the separation and concentration of analytes from an analyte-containing sample, and particularly useful for the separation of unbound tag (or detector) reagents from analyte-bound tag (or detector) reagent. See e.g. U.S. Pat. No. 5,147,529 (Lee). To date, however, there has been no attempt to use magnetic particles in heterogeneous specific binding assays as both a capture agent to bind a target analyte and as a vehicle to move that bound analyte through various reagents and ultimately for detection. Rather, in the past (such as described in connection with the '529 patent) magnetic particles have been placed in a reservoir to bind an analyte, and exposure of an electromagnetic field thereto would cause the magnetic particles to aggregate along the portion of a wall of the reservoir at which the electromagnetic field was applied. By so doing, reagent removal and introduction was facilitated without removing the magnetic particles therefrom, and the assay was conducted with the magnetic particles remaining within the same reservoir.
In addition, Japanese Patent Document JP 43-23,559 is directed to an immunoassay for determining the amount of antigen in a sample using antibody-fixed magnets. These antibody-fixed magnets are said to be arranged so that they can be moved along the depth direction of the wall of a reaction vessel depending on the depth of liquid in the vessel.
Magnetic particles have also been used as precipitating reagents in assays. However, such a use of magnetic particles, like those uses discussed above, is not as a mobile phase for the assay.
It would be desirable for a heterogeneous specific binding assay to use magnetic particles for binding a target analyte in one portion of a device and through the use of an applied potential (which generates an electromagnetic field) to transport the analyte-bound magnetic particles out of that one portion of the device and into another portion of the device so as to render the bound analyte available for detection. And, for reasons of both manufacturing efficiency and health care cost reduction, it would be desirable to perform such heterogeneous specific binding assays in at least a semi-automated manner without compromising the quality or accuracy of the results obtained and the time required to properly conduct the assay.